Electrically driven vehicles are normally equipped with a battery that provides the necessary electric power for operating the vehicle. In this case, the battery may be constructed from a plurality of battery elements or battery cells that each provide a corresponding battery element voltage. One problem for batteries constructed from a plurality of battery elements is even loading of the battery elements during operation. Complex circuits can allow even loading of the battery elements.
Furthermore, changeover between different pairs of battery taps is a critical process that requires appropriate precautions in the circuit design and thereby gives rise to costs. Changeover causes switching losses and, on account of the limited changeover speed, requires complex protective circuitry for the switching elements (for example semiconductors) that channel off overvoltages and maintain the flow of current during changeover. In addition, changeover processes are a main reason for electromagnetic compatibility (EMC) problems as a result of the transmission of electrical waves. Insulated gate electrode bipolar transistors (IGBT), in particular, have a slow switch-off response (what are known as tail currents) and enforce long switching gaps (idle times) in the changeover between multiple pairs of battery taps. In typical high voltage circuits in motor vehicles, which also include motor inverters, for example, switching losses exceed pure resistive losses or line losses in the semiconductors.
In a first approximation, the switching losses are made up of two components. A first component is approximately proportional to the current and voltage during changeover.
On account of the high voltage of several hundred volts (for example 400 V or 800 V), it is additionally possible for capacitive charge reversal effects, for example on the junction capacitances of the semiconductors, to lose significant weight owing to a quadratic dependency on the voltage. The switching losses can be determined by the following formula:
      E          switching      ⁢                          ⁢      loss        =                    ∑                  j          ∈                      Δs            j                              ⁢                          ⁢                                    1            2                    ⁢                      I                          (              j              )                                ⁢                                    V              m                              (                j                )                                      ⁡                          (                                                t                  on                                +                                  t                  off                                            )                                                ︸                      saturation            ⁢                                                  ⁢            losses                                +                            1          2                ⁢                  (                                    C              gd                        +                          C              ds                                )                ⁢                              (                          V              m                              (                j                )                                      )                    2                            ︸                  junction          ⁢                                          ⁢          capacitances                    
DE 10 2011 077 664 A1, which is incorporated by reference herein, discloses an energy storage system having multiple series-connected storage devices and a device for evening the states of charge of the individual storage devices, which comprise at least one DC/DC voltage converter (DC/DC converter).
DE 10 2013 001 466 A1, which is incorporated by reference herein, discloses a battery having multiple battery cells, wherein each battery cell contains a discharge circuit and a supplementary circuit for opening a discharge switch when a critical limit voltage for the respective battery cell is undershot.
DE 10 2014 012 068 A1, which is incorporated by reference herein, discloses a method for heating a battery having a series circuit comprising a plurality of battery cells, wherein a start and an end of the series circuit are coupled via a capacitive store.
EP 2 506 390 A1, which is incorporated by reference herein, discloses a battery controller for a battery having multiple battery cells, wherein each battery cell has an associated circuit for voltage measurement that has a capacitor. The battery controller can change over between the individual battery cells depending on the state of charge.
EP 1 901 412 A2, which is incorporated by reference herein, discloses a battery management system for a battery having multiple battery cells, wherein each battery cell has an apparatus for charging and is charged or discharged depending on the state of charge.
EP 2 053 717 A2, which is incorporated by reference herein, discloses a discharge controller for a battery having multiple battery cells, wherein each battery cell has a discharge circuit and apparatuses for voltage measurement. In addition, a switching apparatus and a control unit are present between the various battery cells.
WO 2013 037 633 A2, which is incorporated by reference herein, discloses a method for equalizing charge differences between battery modules in a battery system. In this case, when the battery system is discharged, energy is stored in an intermediate-circuit capacitor and supplied therefrom to a battery module having a low state of charge.
As can be seen from the formula shown above, current and voltage on a semiconductor at a changeover time are definitive in determining switching losses. If one of these or both is/are negligibly small for individual switching elements at the switching time, switching losses can be effectively reduced, this allowing the switching losses to be kept down even at high switching rates. One object that the present invention addresses is at least a reduction in or even complete elimination of switching losses.